Studies on the mechanism of absorption and distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the rat.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), the toxic contaminant of Agent Orange, is absorbed essentially by the lymphatic route and is transported predominantly by chylomicrons after intestinal absorption. Plasma disappearance of [3H]TCDD-labeled chylomicrons followed first-order decay kinetics, with two exponential components having half-lives of 0.8 and 30 min, respectively. Liver and adipose tissues together accounted for 74 to 81% of the total radioactivity distributed among various tissues. The i.p. route of administration was as effective as the oral route for uptake in 24 h by the adipose and liver tissues, whereas the s.c. route was less efficient. Adipose tissue exhibited a progressive accumulation of labeled TCDD during the first 24 h, whereas the liver showed an exponential disappearance of the newly absorbed TCDD with a half-life of 21.3 h. In contrast, long-term pharmacokinetics of TCDD in the adipose and liver tissues revealed exponential decay patterns with half-lives of 7.6 and 5.3 weeks, respectively. These results show that the adipose tissue and the liver are the major sites of TCDD storage.

Binding, uptake, and metabolism of chylomicron remnants by hepatocytes from control and chronic ethanol-fed rats.

Chylomicron remnants, but not lymph chylomicrons, showed a receptor-dependent high affinity saturable binding to normal rat hepatocytes. The Scatchard analysis of the specific binding data showed a high affinity binding site for the remnants with a dissociation constant of 0.61 nM, assuming a molecular weight of 50 X 10(6) for chylomicron remnants. Based on the heparin-releasable bound radioactivity, approximately 80% of the bound remnants seemed to be internalized. The binding process was markedly inhibited by pronase as well as by protein synthesis inhibitors. Competitive binding studies revealed that the order of competition for the binding of labeled remnants by homologous unlabeled lipoproteins was remnants greater than chylomicrons greater than very low density lipoproteins greater than high density lipoproteins. Human low density lipoproteins showed virtually no competition. Studies on the catabolism of triacylglycerol moiety of the remnants showed that 15.2% of the 14C label in the triacylglycerol moiety of the remnants was catabolized by the hepatocytes to 14CO2 due to specific interaction. This amounted to 93% of the total 14CO2 evolution. This was in sharp contrast to the catabolism of the triacylglycerol moiety of very low density lipoproteins from human and rat, where most of the 14CO2 evolution was due to pathways associated with nonspecific binding. Chronic ethanol feeding caused a 29% (p less than 0.02) decrease in the dissociation constant of the high affinity binding site of the liver cell for the remnants, whereas the extent of internalization was decreased by 19% (p less than 0.01) as compared to the pair-fed control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic ethanol feeding upon the catabolism of protein and lipid moieties of chylomicrons and very low density lipoproteins in vivo and in the perfused heart system.

Effect of chronic ethanol feeding for 6 weeks to male Wistar rats upon the catabolism of rat chylomicrons and very low density lipoproteins was investigated both in vivo and in the perfused heart system. The exponential decay curves in the plasma compartment or in the perfused heart system of these lipoproteins labeled in the protein or triacylglycerol or cholesterol moieties were determined. It was found that chronic ethanol feeding inhibited the catabolism of both protein and triacylglycerol moieties by 26-35%, whereas that of the cholesterol moiety was inhibited by 67-71%. Since the catabolism of the triacylglycerol moiety takes place essentially in the extrahepatic tissues while that of the cholesterol moiety occurs in the liver, it is concluded that ethanol affects the catabolism of triacylglycerol-rich lipoproteins in the liver more than in the extrahepatic tissues.

Relationship of endocrine status on the stimulatory effect of ethanol on hepatic very low density lipoprotein synthesis.

The normal stimulatory effect of an acute oral dose of ethanol on hepatic very low density lipoprotein synthetic rate is abolished in thyroidectomized rats but not in adrenalectomized rats. This lack of stimulation by ethanol can be explained in part by the decreased rate of hepatic fatty acid esterification to neutral lipids in thyroidectomized animals.

Role of high density lipoproteins in regulation of hepatic fatty acid synthesis by chylomicron and very low density lipoprotein remnants.

High density lipoproteins (HDL) have been implicated in the transformation of native triglyceride-rich lipoproteins into their corresponding remnant particles during the action of peripheral lipoprotein lipase. The subsequent metabolism of these remnant particles by the liver results in the inhibition of hepatic lipogenesis. In the present study, remnant particles of chylomicrons or very low density lipoproteins (VLDL) have been generated in the perfused heart system both in the absence and presence of HDL. These have been characterized chemically, and the effects of both native lipoproteins and their respective remnants on fatty acid synthetic rates of hepatocytes have been assessed. Thirty to sixty-six per cent of the triglyceride moieties of native lymph chylomicrons or VLDL were hydrolyzed during a 45-min heart perfusion whether or not HDL was present in the perfusion media. Chylomicron remnants produced in the absence of HDL (25-300 micrograms/ml) caused only 10-20% inhibition of hepatic fatty acid synthesis, whereas remnants produced in the presence of HDL caused up to 78% inhibition at equivalent protein concentrations. The nonsuppressive remnants (produced in the absence of HDL) were converted to suppressive remnants upon incubation with HDL. Similar results were obtained with VLDL remnants produced in the absence and presence of HDL. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis apoprotein profiles of the nonsuppressive and suppressive remnants indicated a marked loss of the C apoproteins during the conversion of native chylomicrons or the nonsuppressive remnants to the suppressive remnants. Thus, HDL seems to be required for the removal of apoprotein C during the transformation of triglyceride-rich lipoproteins to the suppressive remnants. There was, however, no enrichment of apo-E on the suppressive remnant particles. We, thus, could not verify the suggested role of HDL in enriching the suppressive remnants with apoproteins E.

Relationship of alcoholic hyperlipidemia to the feed-back regulation of hepatic cholesterol synthesis by chylomicron remnants.

Pairfeeding of ethanol liquid diet for 6 weeks to male Wistar rats resulted in defective extrahepatic as well as hepatic catabolism of rat lymph chylomicrons. Based upon the exponential decay curves of the chylomicrons in the blood compartment it was concluded that chronic ethanol feeding caused 30 and 67% decreases in the rate of degradation of the triacylglycerol and cholesterol moieties, respectively. As a consequence, abnormal chylomicron remnants accumulated in chronic ethanol-fed but not in control animals. These abnormal remnants were not as efficient as the normal remnants in causing the feedback inhibition of cholesterol synthesis in hepatocytes from normal meal-fed rats. Furthermore, the hepatocytes from chronic ethanol-fed animals exhibited defective feedback regulation of cholesterol synthesis by normal chylomicron remnants. The net result of all these abnormalities caused by chronic ethanol feeding would be the delayed clearance of triacylglycerol-rich lipoproteins from the blood compartment and enhanced synthesis and secretion of the triacylglycerol-rich lipoproteins by the liver.

Alcohol and very low density lipoprotein synthesis and secretion by isolated hepatocytes.

Labeled leucine can be used to measure accurately the rate of both total and secretory protein synthesis by isolated hepatocytes if at least 1 mM leucine is added to the incubation medium, even in the presence of 50 mM ethanol. Using this technique it was found that ethanol caused a significant inhibition of very low density lipoprotein (VLDL) as well as total protein synthetic rates in hepatocytes from both fed and fasted rats. In contrast, a single acute oral dose of ethanol to fasted rats caused within 4 hr a threefold stimulation in the rate of VLDL synthesis without affecting the total protein synthetic rate in the hepatocyte system.

The presence of 2,3-butanediol in the blood of chronic alcoholics admitted to an alcohol treatment center.

Fifty-one of sixty-three unselected alcoholic subjects had 2,3-butanediol in their blood in concentrations ranging from 0.011 to 0.775 mM upon admission to an alcohol detoxification center. The concentration of 2,3-butanediol was below the detection limit of 0.01 mM in twelve non-alcoholic controls. Eighteen hours after admission, second blood samples showed no ethanol in all eleven subjects tested. 2,3-Butanediol levels declined in all of the patients and became undetectable in eight.

Regulation of lipogenesis in isolated hepatocytes by triglyceride-rich lipoproteins.

Very low density lipoproteins, chylomicrons, and remnants caused, within an hour, significant inhibition of fatty acid synthesis but not cholesterol synthesis in hepatocytes isolated from meal-fed rats. In contrast, low density lipoproteins, high density lipoproteins, and the serum fraction of density greater than 1.21 failed to significantly inhibit either fatty acid or cholesterol synthesis within 1 h. The Scatchard plots of specific binding showed that rat and human very low density lipoproteins interact with the high affinity sites on the hepatocytes with the apparent dissociation constants of 64 and 106 nM, respectively. These data also indicated that each hepatocyte was capable of binding 6 X 10(5) molecules of very low density lipoproteins.

Short- and long-term effects of ethanol administration in vivo on rat liver HMG-CoA reductase and cholesterol 7alpha-hydroxylase activities.

Short- and long-term effects of ethanol on HMG-CoA reductase (EC 1.1.1.34) and cholesterol 7alpha-hydroxylase activities in rat liver have been investigated. Neither the reductase nor the hydroxylase activity as measured in vitro was significantly affected within 2 hr after a single intraperitoneal injection of ethanol (7 mmol per 100g body weight), whether tested at the diurnal low or the diurnal high point of activity. Although chronic ethanol feeding for 21 days did not affect the diurnal rhythm of either of these enzyme activities, it caused a 29% decrease in HMG-CoA reductase activity and a 56% decrease in cholesterol 7alpha-hydroxylase activity at the diurnal high point. The same chronic ethanol feeding caused a moderate increase in serum cholesterol and a significant increase in hepatic cholesterol concentration. On the basis of these findings, it is suggested that the decreased rate of cholesterol degradation to bile acids may play a significant role in the accumulation of cholesterol in the liver after chronic ethanol feeding.